1. Field of the Invention
The present invention relates to a digital monitoring system and more particularly, but not by way of limitation, to a digital device for rapid sequential monitoring of a plurality of direct current loops used primarily in security alarm systems.
2. History of the Prior Art
Typically, silent security alarm systems are utilized by businesses and residential homes to monitor the status of various security devices such as door and window switches, electric eyes, thermostat, heat or smoke detectors and the like.
The various security devices are normally connected in series with a direct wire loop which, in turn, is connected to a central monitoring facility. An electrical current is passed through each of these loops and the current level is monitored and compared to a pre-determined current level representing the normal status.
Typically, the current level of each such loop being monitored is by the use of relays, meters, switches, lights and the like provided on each loop. If a security device is tripped, the current level deviation is either manually noted or a relay is tripped thereby lighting a light or sounding an audible alarm to notify the monitoring facility.
Since most direct line security systems utilize leased telephone lines, such lines are constantly in the proximity of electrical transmission lines and electrical appliances. Such direct-wire systems are therefore constantly plagued with 60 Hz line noise which causes false alarms, and generally hampers monitoring operations.
Further, such systems are always susceptible to noise of a random pulse nature such as that caused by lightning or other random static discharges.
The technological advances in digital electronics provides an efficient means of monitoring security alarm systems but past efforts at utilizing high speed digital monitoring systems have been plagued with noise problems of a pulse nature and of the 60 Hz variety requiring that the noise be filtered in the analog form prior to converting to digital form.
Further it was found to be extremely difficult to obtain the resolution necessary since the security equipment varies considerably from one user to another.
Another problem arose in communicating with the user to notify the user that its system was in normal operation when it is turned on.
Typically when the user closes his office for the day, the security equipment is turned on and connected on line. The monitoring facility then manually switches the line to an AC source for a period of time to inform the user that its security system is on line and operating normally. This process is both time consuming and expensive from a man power standpoint.